Electrodeposition of the platinum metals



United States Patent 3,351,541 ELECTRODEPOSHTTON OF THE PLATINUM METALSErnest H. Lyons, In, Marblehead, and Russell M. Dempsey, Hamilton,Mass., assignors to General Electric Company, a corporation of New YorkNo Drawing. Filed Ian. 19, 1965, Ser. No. 426,684

29 Claims. (Cl. Mi l-47) Our invention relates to an improved processand plating bath for electrodepositing a bright, ductile platinum metal.Our invention is additionally directed to a process of obtaining bright,ductile platinum metal deposits on base metals.

A variety of processes are known for electrodepositing platinum metalsto thicknesses of 0.001 inch or less. Roseleurs bath, an aqueoussolution of a complex platino phosphate usually modified with suitablebuffering and brightening agents, is representative of one type of bath.Another type of bath uses ammoniacal dinitrodiammino platinum as asource of the metal to be deposited. A third general type of bath is thealkali platinate bath usually stabilized by the addition of an alkalineoxalate.

Processes of the general types outlined above exhibit a number of Wellrecognized disadvantages. Included in such disadvantages are (1) thedifiiculty in obtaining deposits of any appreciable thickness, (2) thehard, brittle character of the deposits, (3) the necessity ofperiodically replenishing the platinum metal within the plating bath,(4) the necessity of periodic polarity reversal, (5) low currentetficiency, etc.

Atkinson in US. Patent 2,792,341, issued May 14, 1957, discloses aprocedure for electrodepositing bright, ductile platinum. This teachingconstitutes an apparent improvement over previously known platinumdepositing processes, since it purports to overcome the difficulties inobtaining relatively thick, ductile platinum deposits and also toovercome the necessity of periodically replenishing the platinum contentof the bath. One of the distinct disadvantages of the Atkinson processis the requirement that hydrochloric acid be employed in a minimumconcentration of approximately 5 moles, preferably 6.5 moles, and attemperatures above approximately 110 F. and preferably above 150 F. Asrecognized in the patent, the high acid concentration precludes directelectrodeposition on a base metal such as iron, copper, nickel, zinc,silver, etc. Other patent-recognized disadvantages include the tendencyof tungsten and molybdenum to react with hydrochloric acid to forminsoluble acid deposits. Further, the difiiculty, due to evaporation, ofmaintaining concentrated hydrochloric acid at high temperatures wasaffirmed by Atkinson in 1959 (R. H. Atkinson, Vol. 7, Transactions ofthe Institute of Metal Finishing, p. 36) and in 1963 (R. H. Atkinson,Modern Electroplating, p. 316).

It is an object of-our invention to provide an improved process andplating bath for electrodepositing bright, ductile platinum metal.

It is another object or" the invention to deposit bright, ductileplatinum metal without the use of concentrated acids.

It is a further object to provide a process for electrodepositingbright, ductile platinum metal at reduced temperatures.

These and other objects of our invention are accomplished by employingan aqueous plating bath containing in solution at least 100 grams/liter(g./l.) of a lithium or alkaline earth metal halide and from 1 to g./l.of a platinum metal in quadrivalent form, preferably in the form ofplatinum metal hexahalide anions. Electrodeposition of the platinummetal is effected at a cathode,

which may be formed of a base metal, while the anode is preferablyformed of the platinum metal, whereby the bath may be continuouslyreplenished. Deposition is conducted at current densities ranging from10 to 200 milliamperes per square centimeter and at temperatures rangingfrom 30 C. to 110 C.

The term platinum metal as herein employed includes Group VIII metals ofthe light and heavy excepting osmium and iridium. These metals areruthenium, rhodium, palladium, and platinum. The term halide as hereinemployed includes Group VIIA anions with the exception of fluoride. Theincluded halides consist of chlorides, bromides, and iodides. Generally,chlorides are preferred because of their greater availability and lessercost.

The platinum metal is present in the bath in quadrivalent form withsmall amounts of divalent platinum metal also usually being present. Theratio of quadrivalent platinum metal to divalent platinum metal shouldbe greater than 20 to l. A preferred platinum metal compound for use inour plating bath is chloroplatinic acid. Other exemplary additivesinclude chloropalladic acid as Well as chlorides of ruthenium andrhodium. Platinum metal halides may be employed individually or as miX-tures in any proportion. Platinum metals may also be added to the bathin the form of sodium, lithium, or alkaline earth platinum metal halidessuch as, for example, sodium chloroplatinate, lithium chloroplatinate,alkaline earth chloroplatinates, etc. as well as analogs thereof. Otheruseful platinum metal additives include hydrated platinum metal oxidessuch as hydrated platinic oxide for example.

The bath electrolyte is a lithium or alkaline earth metal halide havinga solubility of at least g./l. Lithium halides are generally preferredelectrolytes because of their relatively higher solubilities. Lithiumchloride is a preferred electrolyte. Exemplary suitable alkaline earthmetal halides include magnesium, calcium, strontium, and barium chlorideas Well as analogs thereof. If desired, the bath may be saturated withthe electrolyte.

It is contemplated to use either soluble platinum metal anodes orinsoluble anodes made from material inert to the action of theelectrolyte as, for example, tantalum. When a platinum metal anode isemployed, the anode will enter the bath solution at the rate of metaldeposition, rendering metal replenishment unnecessary. When an inertanode is employed, the platinum metal constituent of the bath may bereplenished by any conventional procedure.

A conventional electrode ode. Whereas acid and plating baths are limitedmay be employed as a cathother highly corrosive electroto the use ofnoble metal electrodes, our invention is practicable with either a nobleor base metal cathode. In other words, our invention may be used toelectroplate metal articles formed from such materials as palladium,gold, silver, nickel, copper, etc., irrespective of whether sucharticles are attacked by strong reagents, such as hydrochloric acid, ornot.

Plating is conducted at temperatures ranging from 30 C. to C. and atcathode current densities of from 10 to 200 ma./cm. The current densityis calculated on the basis of the immersed cathode surface area facingthe anode, as is conventionalpractice. Plating may be conducted in anyconvenient, conventional cell configuration. Since lithium and alkalineearth metal halides do not exhibit the high degree of volatility ofhydrochloric acid and since operating temperatures may be substantiallylower than those employed in corresponding hydrochloric acid platingbaths, only nominal bath maintenance is required.

Plating may be continued for any time duration necessary to build up aplatinum metal deposit of the desired platinum triads,.

thickness. While economic considerations generally dictate thin platinummetal coatings, the process is not limited to the deposition of thincoatings. Unlike conventional platinum metal plating processes, ourprocess may be employed to generate coatings in excess of 1 mil inthickness without resort to ancillary manipulation such as polishing,current reversals, etc.

As is well recognized in the art, the bath should be maintained free ofcontaminants which will adversely infiuence the plating process, as forexample, strong oxidizing agents, strong reducing agents, undesirablebase metals which may co-deposit with the platinum metals, etc.Potassium, ammonium, cesium, rubidium, etc., ions should besubstantially absent from the bath because of the well-known tendency ofthese materials to cause the precipitation of platinum metals asinsoluble salts of the hexahalogeno-platinic acid. Impurities of thetype normally associated with commercial grades of platinum metals arenot, however, considered objectionable to the practice of the invention.

While our process is described in terms of the best mode contemplatedfor its performance, numerous variations will be readily apparent tothose skilled in the art. The process constitutes an improvement overthe process of Atkinson in substituting lithium or alkaline earth metalhalides for hydrochloric acid, however, the invention could, if desired,be practiced using a mixture of hydrochloric acid and metal halides. Insuch instance, the adverse effects of hydrochloric acid areproportionately decreased.

The following examples are illustrative of the practice of our inventionand are not for purpose of limitation:

Example 1 A plating bath is formed consisting essentially of 400 g./l.lithium chloride and 9 g./l. chloroplatinc acid. The bath is maintainedat a temperature of 50 C. A massive platinum anode is immersed in thebath along with a copper cathode in the form of a thin strip. A currentof 62.5 milliamperes per square centimeter is maintained between theelectrodes for a period of approximately 10 minutes.

Upon removal from the bath the copper strip is noted to be coated with abright silver-colored deposit. The strip is bent repeatedly without anydetectable scaling of the coating and without breaking.

Example 2 The procedure of Example 1 is repeated, except that plating iscontinued for 30 minutes. Upon removal from the bath, the copper stripis noted to be coated with a bright, silver-colored deposit. Thethickness of the deposit is visible to the eye and therefore noted to bein excess of 1 mil. The strip is bent repeatedly without breaking orscaling.

Example 3 The procedure of Example 1 is repeated substituting apalladium cathode in strip form for the copper cathode. Upon removalfrom the bath, a barely discernible line of junction between theplatinum deposit and the palladium strip is detected. The platinumcoating is noted to be of a brightness at least equal to that of thepalladium strip. Upon repeated flexing, neither scaling nor breaking isnoted.

Example 4 The procedure of Example 1 is repeated substituting a nickelstrip for the copper cathode. The results achieved substantiallyduplicate the results obtained using a copper cathode.

Example 5 A plating bath is prepared by adding slightly more calciumchloride than is required to form a saturated solution at 80 C. Fiftygrams/liter chloropalladic acid is added. The bath is maintained at 80C.

A palladium metal anode is immersed in the bath along with a coppercathode. A current density of 40 ma./cm. is maintained for a period ofminutes.

Upon removal from the bath, the copper strip is noted to be coated witha bright, silver-colored deposit. The strip is bent repeatedly withoutany detectable scaling of the coating and without breaking.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A process of electroplating a platinum metal comprising preparing anaqueous plating bath containing in solution at least 100 grams/liter ofan electrolyte chosen from the group consisting of lithium halide andalkaline earth metal halide and from 1 to 50 grams/ liter of a platinummetal in quadrivalent form,

maintaining the aqueous plating bath at a temperature of from C. to 110C.,

immersing an anode and a cathode in the plating bath,

and

maintaining a cathode current density of from 10 to 200 milliamperes persquare centimeter. 2. A process according to claim 1 in which saidelectrolyte is lithium chloride.

3. A process according to claim 1 in which said electrolyte is calciumchloride.

4. A process according to claim 1 in which the platinum metal isplatinum.

5. A process according to claim 1 in which the platinum metal ispalladium.

6. A process according to claim 1 in which the platinum metal is presentin the bath in the form of chloroplatinic acid.

7. A process according to claim 1 in which the platinum metal is presentin the bath in the form of chloropalladic acid.

8. A process according to claim 1 in which the anode is platinum.

9. A process according to claim 1 in which the anode is palladium.

10. A process according to claim 1 in which the cathode is a noblemetal.

11. A process according to claim 1 in which the cathode is a base metal.

12. A process of electroplating a platinum metal comprising preparing anaqueous plating bath containing in solution at least 100 grams/liter ofan electrolyte chosen from the group consisting of lithium chloride andalkaline earth metal chloride and from 1 to grams/liter of a platinummetal in the form of divalent platinum metal hexachloride anions,

maintaining the aqueous plating bath at a temperature of from 30 C. to110 C.,

immersing an anode and a cathode in the plating bath,

and

maintaining a current density between the anode and cathode of from 10to 200 mlliamperes per square centimeter.

13. A process according to claim 12 in which the platinum metal ispresent in the bath in the form of chloroplatinic acid and the anode isplatinum.

14. A process according to claim 12 in which the platinum metal ispresent in the bath in the form of chloropalladic acid and the anode ispalladium.

15. A process according to claim 12 in which the cathode i a base metal.

16. An aqueous plating bath containing in solution at least grams/literof an electrolyte chosen from the group consisting of lithium halide andalkaline earth metal halide and from 1 to 50 grams/liter of platinummetal in quadrivalent form.

17. A plating bath according to claim 16 in which the platinum metal ispresent in the form of'divalent platinum metal hexahalide anions.

18. A plating bath according to claim 17 in which the platinum metal ispresent in the bath in the form of chloroplatinic acid.

19. An aqueous plating bath containing in solution at least 100grams/liter of an electrolyte chosen from the group consisting oflithium chloride and alkaline earth metal chloride and from 1 to 50grams/liter of a platinum metal in the form of divalent platinum metalhexachloride anions.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner.

20. An aqueous plating bath according to claim 19 in 10 G. KAPLAN,Assistant Examiner.

which the platinum metal is platinum.

1. A PROCESS OF ELECTROPLATING A PLATINUM METAL COMPRISING PREPARING ANAQUEOUS PLATING BATH CONTAINING IN SOLUTION AT LEAST 100 GRAMS/LITER OFAN ELECTROLYTE CHOSEN FROM THE GROUP CONSISTING OF LITHIUM HALIDE ANALKALINE EARTH METAL HALIDE AND FROM 1 TO 50 GRAMS/ LITER OF A PLATINUMMETAL IN QUADRIVALENT FORM, MAINTAINING THE AQUEOUS PLATING BATH AT ATEMPERATURE OF FROM 30*C. TO 110*C., IMMERSING AN ANODE AND A CATHODE INTHE PLATING BATH, AND MAINTAINING A CATHODE CURRENT DENSITY OF FROM 10TO 200 MILLAMPERES PER SQUARE CENTIMETER.